Polyesters, such as polyethylene terephthalate (PET) are prepared industrially in a two stage process. The first stage in PET preparation involves the direct esterification of terephthalic acid with ethylene glycol, or alternatively transesterification of a C1-C4 dialkylterephthalate with ethylene glycol to form a low molecular weight precondensate. In a second stage, the precondensate is polycondensed to form high molecular weight polyethylene terephthalate. Both stages typically employ catalytic acceleration.
Depending on the end use of the polyester, a further solid state polymerization (SSP) is employed to arrive at the desired viscosity.
Numerous compounds have been proposed as esterification, transesterification or polycondensation catalysts. Choice of catalyst effects the color, strength and processing properties of the end product. Choice of catalyst effects for example the amount of aldehyde generation. Catalyst selection also controls selectivity of the reaction and provides for final products reduced in unwanted impurities such as diethylene glycol, cyclic oligomers and carboxylic acid end groups.
U.S. published app. No. 2003083191 is aimed at polyester polymerization catalysts that are combination of aluminum or an aluminum compound and a phosphorus metal salt compound. The aluminum compounds are for example carboxylates, inorganic acid salts, aluminum alkoxides, aluminum chelate compounds or organoaluminum compounds.
U.S. published app. No. 2004058805 is aimed at polyester polymerization catalysts that comprise at least one member selected from aluminum and aluminum compounds as a first metal-containing component in the presence of at least one phosphorus compound selected from formula 1 and 2.

U.S. Pat. Nos. 3,310,575 and 3,824,192 disclose metal (O-alkyl)-3,5-di-t-butyl-4-hydroxybenzylphosphonates. The metal phosphonates are taught as polymer stabilizers.
U.S. Pat. No. 6,383,970 teaches solid acid catalysts.
TW 526235 discloses a production process for polyester resin.
Suitable catalysts for the preparation of polyesters, in particular polyethylene terephthalate are still being sought. Said catalysts are employed in the first or second stages of preparation, or in both stages.